Fix gunner goggles

[kymckay]

Fixes #1721 and #1342

Originally I introduced a canUseWeapon check to fix the FFV seats. Then when it came to considering the second issue I think it makes sense to just remove any special treatment for gunners anyway:

• It simplifies the code and offers a slight performance benefit (that function is used by a PFH)
• If you intend to be a vehicle gunner, you probably shouldn't be wearing unsuitable glasses anyway. Besides it's not the end of the world if you do, you can still see after all.
• It makes things more consistent
  • Overlay already persists when using a UAV
  • Would be inconsistent for the weirdly configured commander seats/co-pilot seats
  • All the other effects checked by that function will now still take place when in a vehicle (if the vehicle goes underwater/etc.)
• We will be introducing a client setting for the goggles overlay so nobody is forced to use it by default.

[kymckay]

Labelled discussion as this contains a minor design decision that others might want to weigh in on

[PabstMirror]

You can access your inventory while mounted, so this should be fine.

[kymckay]

No objections in the past 48 hours, merging

[CorruptedHeart]

I'm going to weigh in on this a bit late. But the original reason for it was due to the inventory not being accessible in a vehicle.

So now that, that is possible. No reason to keep it around.